1. Field of the Invention
The present invention generally relates to solid state imagers and, more particularly, to a horizontal transfer section of a solid state imager in which charge transfer efficiency of its horizontal transfer section is improved.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows an arrangement of a CCD (charge-coupled device) solid state imager of, for example, an interline transfer type as an example of conventional solid state imagers.
As shown in FIG. 1, an image pickup section 3 is comprised of a plurality of sensitive units (pixels) 1 arrayed in the horizontal and vertical directions in a two-dimensional manner so as to store signal charges corresponding to the amount of an incident light. A vertical shift register (vertical transfer sections) 2 is provided for transferring signal charges read-out from these sensitive sections 1 at every vertical column in the vertical direction. In the image pickup section 3, the sensitive section 1 is made of, for example, a photodiode, and the vertical shift register 2 is made of a CCD. Signal charges transferred to the vertical shift register 2 are sequentially transferred to a horizontal shift register (i.e., horizontal transfer section) 4 by an amount corresponding to one scanning line during a part of the horizontal blanking period. Signal charges corresponding to the amount of one scanning line are sequentially transferred by the horizontal shift register 4 in the horizontal direction. An output circuit section 5 is provided at the end of the horizontal shift register 4. This output circuit section 5 is formed of a floating diffusion amplifier (i.e., FDA) to convert a signal charge transferred thereto into an electrical signal.
Of this kind of CCD solid state imager, particularly in a device utilizing a semiconductor substrate of a first conductivity type, e.g., N type, respective elements are provided on a P-well region. This P-well region is formed in a depletion state so as to function as an overflow barrier in order to remove a so-called blooming in the image sensor section. As to the horizontal shift register 4, in order to match the drive condition of a transfer clock for driving the horizontal transfer section, i.e., in order to match the voltage level (e.g., voltages of 5 V and 0 V) of a horizontal transfer clock, a potential of a P well-region is decreased by the implantation of boron (B) therein.
In this case, however, since the potential of the P well-region is lowered or made shallow by the implantation of boron (B) to the P well-region, it is to be understood that, when the horizontal transfer clock is "L" (low) in level, the P well-region is not formed in a depletion state. As a result, the potential of the P well-region becomes neutral, thus resulting in the charge transfer efficiency of the horizontal transfer section being deteriorated.